fix(opencode): handle signature writer errors

agent/opencode/opencode_model_test.go

@@ -3,6 +3,7 @@ package opencode
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -14,6 +15,12 @@ import (
 	"github.com/chenhg5/cc-connect/core"
 )
 
+type errWriter struct{}
+
+func (errWriter) Write(_ []byte) (int, error) {
+	return 0, errors.New("write failed")
+}
+
 // writeFakeModelsBin writes a temporary shell script that acts as a fake CLI.
 // When invoked with "models", it prints lines to stdout.
 // When exitCode != 0, the script exits immediately with that code.
@@ -40,6 +47,16 @@ func writeFakeModelsBin(t *testing.T, lines []string, exitCode int) string {
 	return name
 }
 
+func TestWriteProviderSignaturePart_PropagatesWriterError(t *testing.T) {
+	err := writeProviderSignaturePart(errWriter{}, "name", "value")
+	if err == nil {
+		t.Fatal("writeProviderSignaturePart() error = nil, want write failure")
+	}
+	if !strings.Contains(err.Error(), "write failed") {
+		t.Fatalf("writeProviderSignaturePart() error = %v, want write failure", err)
+	}
+}
+
 func writePersistentModelCache(t *testing.T, cachePath string, models []core.ModelOption, updatedAt time.Time) string {
 	t.Helper()